1. Technical Field
The invention relates to an optical scanning device for scanning a first type of record carrier provided with a first information layer and a first transparent layer having a first thickness, and for scanning a second type of record carrier provided with a second information layer and a second transparent layer having a second thickness which differs from the first thickness.
2. Related Art
Generally, the transparent layer in optical record carriers is intended for protecting the information layer from ambient influences and for providing mechanical support to the information layer, in other words, the transparent layer functions as a substrate for the information layer. The thickness of the transparent layer is a compromise between the thickness which is desired to give the record carrier the desired rigidity and the thickness which is desired in connection with the numerical aperture (NA) of the scanning beam incident on the transparent layer. The NA of the objective system on the side of the record carrier is determined by the resolution which the scanning device must have, i.e. by the information density of the information layer. Generally, a higher NA is necessary for a larger information density. For envisaged novel record carriers with a larger information density thus requiring a higher NA, it is often necessary to reduce the thickness of the information layer so as to decrease the influence of tilt of the record carrier on the quality of the focus, or scanning spot. In fact, this influence will be greater at a higher NA. With the advent of novel record carriers having larger information densities, different types of record carriers having different thicknesses of the transparent layer will be on the market. A compatible scanning device will have to be able to scan the different types of record carriers, independently of the thickness of the transparent layer. Scanning a record carrier is herein understood to mean the movement of a scanning spot formed by the scanning beam and the information layer relative to each other for the purpose of reading, writing and/or erasing information.
The transparent layer, through which the scanning beam passes, introduces spherical aberration in the scanning beam. In the design of the objective system, the transparent layer can be taken into account so that this system can compensate said spherical aberration. However, since a given objective system can compensate only for a given thickness of the transparent layer, the quality of the scanning spot will deteriorate due to the under or overcompensated spherical aberration when using this objective system for scanning a record carrier having a different thickness, as is envisaged for a compatible scanning device.
U.S. Pat. No. 5,708,638 (PHN 15.724) discloses a compatible scanning device designed for scanning a first type of record carrier having a larger information density, for example, a record carrier known as DVD. To preclude the detrimental influence of spherical aberration when scanning a second type of record carrier having a smaller density, for example, a record carrier known as CD, it has been ensured that the detector receives only radiation coming from the central part of the pupil, i.e. the central part of the scanning beam coming from this record carrier. When scanning the first type of record carrier, the detector receives radiation of the entire scanning beam coming from this record carrier. These conditions can be fulfilled by adapting, for example, the size of the radiation-sensitive surface of the detector in such a way that, when scanning the first type of record carrier, this size is equal to the cross-section of the scanning beam at the location of the detector, whereas the radiation-sensitive surface comprises only the central part of the beam cross-section of the scanning beam coming from the second record carrier when scanning the second type of record carrier. Use is made of the fact that the spherical aberration, which occurs when scanning the second type of record carrier, mainly occurs in the peripheral part of the scanning beam. By detecting only radiation coming from the central part of the pupil diameter, which central part is, for example, 55% of the pupil diameter, and not detecting the peripheral part of this beam, it is achieved that the beam is reasonably free from spherical aberration for this detector, while this beam still has a sufficient intensity to supply satisfactory signals.
The detector does not only supply an information signal, which represents the information read from the record carrier, but also a focus error signal. The latter signal, which is representative of an axial deviation between the focal plane of the objective system and the plane of the information layer, is used to correct the axial position of the focus, for example, by displacing the objective system in the axial direction. A focus error signal may be obtained, for example, by providing an astigmatizing element, for example, a cylindrical lens in the path of only the reflected scanning beam between the objective system and the detector, which element converts the beam into an astigmatic beam. Such a beam has the property that the shape of the radiation spot formed in the plane of the detector changes upon an axial displacement of the focus of the scanning spot with respect to the information layer. This change of shape can be observed with a detector consisting of four separate detector elements. By combining the output signals of these detector elements in a given way, a focus error signal, referred to as the astigmatic focus error signal, can be obtained. However, it has been found that the desired focus error signal is not obtained when using the astigmatic focus detection method in a compatible scanning device with the above-mentioned small detector surface for scanning the second type of disc. Instead of a smooth S curve having a steep slope around zero, the graph of the focus error signal has a jagged variation and a faint slope around zero, so that the focus error signal obtained is not very well usable in practice.
It is an object of the invention to provide an optical scanning device of the type described in the opening paragraph, in which a suitable focus error signal is obtained when scanning the second type of record carrier without this being at the expense of the signals obtained when scanning the first type of record carrier. To this end, the scanning device according to the invention is characterized in that an aspherical surface having a size which is substantially equal to the cross-section of said central part is present in the path of only the modulated scanning beam between the objective system and the detector.
The invention is based on the recognition that, when scanning the second type of record carrier, radiation of the scanning beam outside the central part of the pupil diameter is still partly incident on the quadrant detector. Since this beam portion exhibits spherical aberration, this detector detects a beam which is not only astigmatic but also exhibits spherical aberration. Consequently, the radiation spot formed on the detector no longer has a well-detectable change of shape when focus errors occur. By providing an extra aspherical profile in the detection branch of the scanning device, i.e. in the path of only the beam coming from the record carrier and only in the central part of the pupil diameter, this spherical aberration is compensated. Consequently, the beam received by the radiation-sensitive surface of the detector is sufficiently free from aberration to derive a satisfactory focus error signal, also when scanning the second type of record carrier.
An embodiment of the scanning device, in which an astigmatic element is arranged in the path of only the modulated scanning beam, is further characterized in that the aspherical surface is constituted by a surface of the astigmatic element.
For correcting the spherical aberration, it is then not necessary to place an extra element in the detector branch, so that material costs and assembly time can be saved. The astigmatic element, which may be a separate cylindrical lens but may be alternatively constituted by another component of the scanning device, is necessary to render the modulated scanning beam astigmatic, so that a focus error signal can be derived from this beam.
A preferred embodiment of the scanning device, in which the astigmatic element is constituted by a cylindrical lens and in which the cylindrical lens is integrated with a rotationally symmetric lens, is characterized in that the aspherical surface is constituted by a surface of the rotationally symmetric lens.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.